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Before this CL, MainController calls startUpChromeUI on the scene on iOS
12.

With this CL, this exceptional call is removed. Instead, the call now
always happens in SceneController. The follow-up call to
startUpAfterFirstWindowCreated in MainController is now triggered
through the appState:firstSceneActivated: observer callback in both old
and new startup sequences.

This requires a few minor changes:
* AppState now observes SceneWillConnect and immediately adds itself
as a SceneState observer. The UISceneDidActivateNotification observation
is now replaced with sceneState:transitionedToActivationLevel:, which
is available on both iOS 12 and 13.
* SceneController now becomes an AppStateObserver immediately on init,
instead of waiting to initializeUI.

Minor win: SceneControllerGuts are no longer necessary.

Bug: 1012697
Change-Id: Id82e6a10c5825e34f53f076fef19a1e6bf9e7908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398505
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807938}